1. Field of the Invention
The present invention generally relates to a single side band modulation device for single side band transmission, and in particular, to a single side band modulation device capable of simultaneously transmitting carrier channels.
2. Description of the Related Art
The spectrum of a modulated signal obtained by modulating amplitude-modulated data to a carrier forms an upper side band and a lower side band with respect to a carrier frequency serving as an axis of symmetry. For transmission of the modulated signal, single side band transmission is used where the carrier and one of those side bands are removed and only the other one is used.
The upper side band is a high frequency band to which the channels of the modulated signal are shifted and the lower side band is a low frequency band to which the inverted channels of the modulated signal are shifted.
Single side band transmission requires a bandwidth that is one half that of double side band transmission. By not transmitting a carrier and a side band, amplifier power consumption is minimized. In addition, single side band transmission reduces noise due to its narrow bandwidth, thereby improving a signal-to-noise ratio and reception sensibility. A single side band modulation device for single side band transmission filters in an optical domain using an optical fiber Bragg grating. However, the single side band modulation device is not suitable for actual use due to the difficulty in manufacturing the optical fiber Bragg grating and its poor stability. To solve the problem of the existing single side band modulation device, a LiNbO3-based single side band modulation device having superior stability is suggested.
FIG. 1 illustrates a conventional single side band modulation device 100 for single side band transmission. Referring to FIG. 1, the single side band modulation device 100 includes a single side band modulation module 120, a hybrid coupler 130, and a light source 110. The light source 110 generates carriers. The hybrid coupler 130 processes input data into a first signal having a phase of 0° and a second signal having a phase of 90° and outputs the first signal and the second signal to the single side band modulation device 120.
FIG. 2 illustrates the single side band modulation module 120 illustrated in FIG. 1. Referring to FIG. 2, the single side band modulation module 120 includes first through fifth phase modulators 121a, 121b, 122a, 122b, and 123. The first phase modulator 121a and the second phase modulator 121b constitute a first Mach Zehnder interferometer 121, and the third phase modulator 122a and the fourth phase modulator 122b constitute a second Mach Zehnder interferometer 122. The first signal and the second signal are input to each of the first Mach Zehnder interferometer 121 and the second Mach Zehnder interferometer 122 from the hybrid coupler 130 in a push-pull manner.
The first Mach Zehnder interferometer 121 generates a third signal by mixing the first signal and the second signal that are push-pull input from the hybrid coupler 130 and the second Mach Zehnder interferometer 122 generates a fourth signal by mixing the first signal and the second signal. The first through fourth signals have phase differences of 0 and π by bias voltages applied to the first through fifth phase modulators 121a, 121b, 122a, 122b, and 123. In particular, the phase of the third signal is +π/2 or −π/12 shifted by the fifth phase modulator 123.
FIGS. 3A through 3G are graphs illustrating changes in the phases of the first signal and the second signal in the single side band modulation module 120. The graphs show changes in the arrangement of a Bessel function with respect to the changes in the phases of the first signal and the second signal in each component of the single side band modulation module 120 when cos(ωt) is input to the hybrid coupler 130. J in graphs (a) through (g), i.e., FIGS. 3A to 3G, indicates a first kind Bessel function. Subscripts 0, 1, 2, 3 of J indicate the orders of the Bessel function. In other words, J0(x) is a first kind zero-order Bessel function, J1(x) is a first kind first-order Bessel function, J2(x) is a first kind second-order Bessel function, and J3(x) is a first kind third-order Bessel function.
The independent variable x is determined by a voltage applied to the single side band modulation module 120 and an inherent switching voltage of the single side band modulation module 120 (a voltage that should be applied for 180° phase shift). As the order of a first kind Bessel function increases with respect to the same factor x, the magnitude of the first kind Bessel function decreases. Thus, first kind Bessel functions whose orders are greater than 3 will be omitted. In FIG. 3, the size of an arrow indicates the magnitude of the first kind Bessel function and the direction of an arrow indicates a phase. In other words, when the Y-axis direction is assumed to be a phase of 0, the X-axis direction indicates a phase of π/2, the −Y-axis direction indicates a phase of π, and the −X-axis direction indicates a phase of 3π/2. The Z-axis indicates a frequency. With respect to the J0 carrier frequency, the frequency ±ω is applied to result in J1, the frequency ±2ω (which is 2 times the applied frequency) is used to result in J2, and a frequency ±3ω (which is 3 times the applied frequency) is used to result in J3.
The graphs (a) and (b) respectively show the waveforms of the first signal and the second signal that are push-pull input to the first Mach Zehnder interferometer 121, the graphs (c) and (d) respectively show the waveforms of the first and second signals that are push-pull input to the second Mach Zehnder interferometer 122, and the graph (e) shows the waveform of the third signal obtained by mixing the first signal and the second signal to the first Mach Zehnder interferometer 121. The third signal is mixed with the waveform of the fourth signal shown in the graph (f) after being ±π/2 phase-modulated by the fifth phase modulator 123.
When the third signal is +π/2 phase-modulated, it is mixed with the fourth signal to form an upper side band (USB) signal. When the third signal is −π/2 phase-modulated, it is mixed with the fourth signal to form a lower side band (LSB) signal. The resulting USB and LSB signals are represented in graph (g).
However, it can be seen from the graph (g) that a carrier frequency is removed from both the upper side band signal and the lower side band signal. Referring to FIGS. 4A, 4B, 4C, 4D showing changes in eye-diagrams with respect to the magnitude of a carrier frequency, it can be seen that the graph in FIG. 4A showing an eye-diagram with respect to a carrier frequency of the smallest magnitude is smallest and the graph in FIG. 4D showing an eye-diagram with respect to a carrier frequency of the largest magnitude is largest and represented clearly.
Modulation of a single side band from which a carrier frequency is removed cannot use a receiver of a direction detection type, but should use a complicated detector of an optical interferometer type instead.
To solve the problem, an offset may be applied to a conventional single side band modulation device. However, in this case, an unwanted other side band may be mixed. In other words, a lower side band signal may be mixed during transmission of an upper side band signal or the upper side band signal may be mixed during transmission of the lower side band signal.